1. Field of the Invention
The present invention relates to a method of fabricating a part for use in a nuclear reactor which part is exposed to neutron flux in use, and in particular, but not exclusively, to a method of fabricating a channel box for a fuel assembly to be loaded in the core of a boiling-water reactor.
2. Description of the Prior Art
FIGS. 1 and 2 of the accompanying drawings show the shape of a conventional channel box of a fuel assembly for a boiling-water reactor. This channel box is also known as a channel. It consists of a square-section tubular body 11 with rounded corners made by butt welding two previously bent zirconium alloy plates 11a. In section, as shown in FIG. 2, the four corners and the four sides joining them have the same uniform thickness. At the upper end of the channel box there are provided the conventional welded-on additions, which are a clip 12 with a hole for use in locating and fixing the channel box on the upper tie plate of the fuel assembly, a clip 13 without a hole at the diagonally opposite corner, and on the outside of the channel box on two sides channel spacers 14. The spacers 14 face the control rod space in the core of the reactor, and serve to maintain the gaps between the channel boxes of adjacent fuel assemblies
FIG. 3 shows a method of fabricating the channel box of FIGS. 1 and 2, this process being generally similar to what is disclosed in U.S. Pat. No. 4,238,251. Two flat plates 1 of zirconium alloy are bent to form two U-shaped pieces 11a, which are then butt welded to form the tubular body 11. The body 11 then undergoes solution heat treatment including a quenching step from a temperature at which the zirconium alloy has a .beta.-phase, in order to provide high corrosion resistance to the material. Thereafter, there is a sizing step to achieve accuracy of size of the channel box, and finally a finishing step in which the clips 12, 13 and spacers 14 are fitted.
U.S. Pat. No. 4,238,251 mentioned above particularly describes the solution heat treatment process, whose function is to give the material high corrosion resistance. This solution heat treatment process involves heating the zirconium alloy to a region at which the .beta.-phase exists, which may be the .beta.-phase region in its phase diagram or the region of the .alpha.-phase and .beta.-phase in the phase diagram, followed by quenching rapidly to below the phase transition temperature.
A further disclosure of solution heat treatment processes which may be employed in the present invention is U.S. Pat. No. 4,678,521 (corresponding to JP-B-61-45699) which is primarily a disclosure of a process for producing the flat zirconium alloy plate 1 shown in FIG. 3. However, the process includes solution heat treatment steps, which may be employed in the present invention (see particularly column 7, lines 31-42).
Because of its exposure to the neutron flux in the reactor core, the channel box tends to suffer from bulging of its sides, due to creep expansion. It also suffers from embrittlement, oxidation and elongation. To reduce the problem of bulging, at least by extending the useful life of the channel box, it has been proposed to form the channel box with thin side portions 19 between thicker corner portions 20, as shown in present FIGS. 4 and 5 (JP-B-1-13075). Typically the thinner side portions 19 are thinner than the uniform thickness of the channel box of FIG. 1, and the thicker corner portions 20 are thicker than the uniform thickness of the channel 11 of FIG. 1. The effect is to provide greater clearance between the thin portions 19 and the space occupied by the control rod in the reactor core, so that the tolerance for bulging is greater.
One process for making the channel box of FIG. 4 is disclosed in U.S. Pat. No. 4,749,543 and U.S. Pat. No. 4,749,544 (corresponding to JP-A-63-253290). This process involves forming eight zirconium alloy parts, comprising four corner parts and four thin side parts, and welding these together along lines extending longitudinally of the channel box. The thin side parts may have thinner grooves extending part of the length of the channel box, formed by machining.
Another process for fabricating the channel box of FIG. 4 is disclosed in JP-A-1-227991, in which zirconium alloy sheets are first rolled between special rolls, one of which has larger and smaller diameter portions to form thicker and thinner portions of the plates corresponding to corners and side portions of the channel box, after which the corner portions are bent to give two U-section pieces. These two pieces are then welded to form the channel box 15. It appears more convenient to form these U-section pieces in shorter lengths than the full length of the channel box, and to weld these lengths together to form a full length piece, prior to welding longitudinally.
The disadvantage of the fabricating process of U.S. Pat. No. 4,749,544 is the complexity of preparing four corner parts and four thin side parts and welding these together along eight lines, which is not only time-consuming but also makes quality control difficult, as compared with the process of FIG. 3 where two welds are required. U.S. Pat. No. 4,749,544 does not disclose any heat treatment to improve corrosion resistance.
The process of JP-A-1-227991 is disadvantageous, because it is difficult to roll long plates having thinner and thicker portions in zirconium alloy, and as mentioned it is necessary to sub-divide the parts longitudinally, and then weld them together.
Although it is not published, it can be mentioned here that the present inventors have attempted to apply the process of solution heat treatment described above, to the zirconium alloy channel box having side portions of reduced thickness, i.e. the channel box of FIG. 4. It is not difficult to subject the channel box 11 of FIG. 1 to solution heat treatment, to achieve high corrosion resistance, but difficulties arise in applying the same process to the channel box of FIG. 3 having different wall thicknesses, so as to achieve circumferentially uniform solution heat treatment. The same problem arises if the body has a longitudinal variation of wall thickness.